112 research outputs found
J-type Carbon Stars in the Large Magellanic Cloud
A sample of 1497 carbon stars in the Large Magellanic Cloud has been observed
in the red part of the spectrum with the 2dF facility on the AAT. Of these, 156
have been identified as J-type (i.e. 13C-rich) carbon stars using a technique
which provides a clear distinction between J stars and the normal N-type carbon
stars that comprise the bulk of the sample, and yields few borderline cases. A
simple 2-D classification of the spectra, based on their spectral slopes in
different wavelength regions, has been constructed and found to be related to
the more conventional c- and j-indices, modified to suit the spectral regions
observed. Most of the J stars form a photometric sequence in the K - (J-K)
colour magnitude diagram, parallel to and 0.6 mag fainter than the N star
sequence. A subset of the J stars (about 13 per cent) are brighter than this J
star sequence; most of these are spectroscopically different from the other J
stars. The bright J stars have stronger CN bands than the other J stars and are
found strongly concentrated in the central regions of the LMC. Most of the
rather few stars in common with Hartwick and Cowley's sample of suspected CH
stars are J stars. Overall, the proportion of carbon stars identified as J
stars is somewhat lower than has been found in the Galaxy. The Na D lines are
weaker in the LMC J stars than in either the Galactic J stars or the LMC N
stars, and do not seem to depend on temperature.Comment: 19 pages, 21 figures, Latex; in press, MNRA
Structure of the Large Magellanic Cloud from 2MASS
We derive structural parameters and evidence for extended tidal debris from
star count and preliminary standard candle analyses of the Large Magellanic
Cloud based on Two Micron All Sky Survey (2MASS) data. The full-sky coverage
and low extinction in K_s presents an ideal sample for structural analysis of
the LMC.
The star count surface densities and deprojected inclination for both young
and older populations are consistent with previous work. We use the full areal
coverage and large LMC diameter to Galactrocentric distance ratio to infer the
same value for the disk inclination based on perspective.
A standard candle analysis based on a sample of carbon long-period variables
(LPV) in a narrow color range, 1.6<J-K_s<1.7 allows us to probe the
three-dimensional structure of the LMC along the line of sight. The intrinsic
brightness distribution of carbon LPVs in selected fields implies that
\sigma_M\simlt 0.2^m for this color cut. The sample provides a {\it direct}
determination of the LMC disk inclination: .
Distinct features in the photometric distribution suggest several distinct
populations. We interpret this as the presence of an extended stellar component
of the LMC, which may be as thick as 14 kpc, and intervening tidal debris at
roughly 15 kpc from the LMC.Comment: 24 pages, 9 figures. Submitted to Ap
Review of important reactions for the nitrogen chemistry in the interstellar medium
Predictions of astrochemical models depend strongly on the reaction rate
coefficients used in the simulations. We reviewed a number of key reactions for
the chemistry of nitrogen-bearing species in the dense interstellar medium and
proposed new reaction rate coefficients for those reactions. The details of the
reviews are given in the form of a datasheet associated with each reaction. The
new recommended rate coefficients are given with an uncertainty and a
temperature range of validity and will be included in KIDA
(http://kida.obs.u-bordeaux1.fr).Comment: 39 pages, not published in refereed journal, datasheets are given in
KID
Hipparcos period-luminosity relations for Miras and semiregular variables
We present period-luminosity diagrams for nearby Miras and semiregulars,
selecting stars with parallaxes better than 20 per cent and well-determined
periods. Using K-band magnitudes, we find two well-defined P-L sequences, one
corresponding to the standard Mira P-L relation and the second shifted to
shorter periods by a factor of about 1.9. The second sequence only contains
semiregular variables, while the Mira sequence contains both Miras and
semiregulars. Several semiregular stars show double periods in agreement with
both relations. The Whitelock evolutionary track is shown to fit the data,
indicating that the semiregulars are Mira progenitors. The transition between
the two sequences may correspond to a change in pulsation mode or to a change
in the stellar structure. Large amplitude pulsations leading to classical Mira
classification occur mainly near the tip of the local AGB luminosity function.Comment: 10 pages with figures, accepted by ApJ Letter
Reprocessing the Hipparcos data for evolved stars III Revised Hipparcos period-luminosity relationship for galactic long-period variable stars
We analyze the K band luminosities of a sample of galactic long-period
variables using parallaxes measured by the Hipparcos mission. The parallaxes
are in most cases re-computed from the Hipparcos Intermediate Astrometric Data
using improved astrometric fits and chromaticity corrections. The K band
magnitudes are taken from the literature and from measurements by COBE, and are
corrected for interstellar and circumstellar extinction. The sample contains
stars of several spectral types: M, S and C, and of several variability
classes: Mira, semiregular SRa, and SRb. We find that the distribution of stars
in the period-luminosity plane is independent of circumstellar chemistry, but
that the different variability types have different P-L distributions. Both the
Mira variables and the SRb variables have reasonably well-defined
period-luminosity relationships, but with very different slopes. The SRa
variables are distributed between the two classes, suggesting that they are a
mixture of Miras and SRb, rather than a separate class of stars. New
period-luminosity relationships are derived based on our revised Hipparcos
parallaxes. The Miras show a similar period-luminosity relationship to that
found for Large Magellanic Cloud Miras by Feast et al. (1989). The maximum
absolute K magnitude of the sample is about -8.2 for both Miras and
semi-regular stars, only a little fainter than the expected AGB limit. We show
that the stars with the longest periods (P>400d) have high mass loss rates and
are almost all Mira variables.Comment: Comments welcome. Submitted to A&A 11 pages, 7 figs, 3 table
s-Process Nucleosynthesis in Carbon Stars
We present the first detailed and homogeneous analysis of the s-element
content in Galactic carbon stars of N-type. Abundances of Sr,Y, Zr (low-mass
s-elements, or ls) and of Ba, La, Nd, Sm and Ce (high-mass s-elements, hs) are
derived using the spectral synthesis technique from high-resolution spectra.
The N-stars analyzed are of nearly solar metallicity and show moderate
s-element enhancements, similar to those found in S stars, but smaller than
those found in the only previous similar study (Utsumi 1985), and also smaller
than those found in supergiant post-AGB stars. This is in agreement with the
present understanding of the envelope s-element enrichment in giant stars,
which is increasing along the spectral sequence M-->MS-->S-->SC-->C during the
AGB phase. We compare the observational data with recent -process
nucleosynthesis models for different metallicities and stellar masses. Good
agreement is obtained between low mass AGB star models (M < 3 M_o) and
s-elements observations. In low mass AGB stars, the 13C(alpha, n)16O reaction
is the main source of neutrons for the s-process; a moderate spread, however,
must exist in the abundance of 13C that is burnt in different stars. By
combining information deriving from the detection of Tc, the infrared colours
and the theoretical relations between stellar mass, metallicity and the final
C/O ratio, we conclude that most (or maybe all) of the N-stars studied in this
work are intrinsic, thermally-pulsing AGB stars; their abundances are the
consequence of the operation of third dredge-up and are not to be ascribed to
mass transfer in binary systems.Comment: 31 pages, 10 figures, 6 tables. Accepted in Ap
Hipparcos red stars in the HpV_{T2} and VI_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibrated
instantaneous (epoch) Cousins color indices using newly derived
photometry. Three new sets of ground-based Cousins data have
been obtained for more than 170 carbon and red M giants. These datasets in
combination with the published sources of photometry served to obtain the
calibration curves linking Hipparcos/Tycho with the Cousins
index. In total, 321 carbon stars and 4464 M- and S-type stars have new
indices. The standard error of the mean is about 0.1 mag or better down
to although it deteriorates rapidly at fainter magnitudes. These
indices can be used to verify the published Hipparcos color
indices. Thus, we have identified a handful of new cases where, instead of the
real target, a random field star has been observed. A considerable fraction of
the DMSA/C and DMSA/V solutions for red stars appear not to be warranted. Most
likely such spurious solutions may originate from usage of a heavily biased
color in the astrometric processing.Comment: 10 figures, 1 electronic table, accepted in A&
Astrometry with Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator RR Lyrae
We present an absolute parallax and relative proper motion for the
fundamental distance scale calibrator, RR Lyr. We obtain these with astrometric
data from FGS 3, a white-light interferometer on HST. We find mas. Spectral classifications and VRIJHKTM and DDO51 photometry of
the astrometric reference frame surrounding RR Lyr indicate that field
extinction is low along this line of sight. We estimate =0.07\pm0.03 for
these reference stars. The extinction suffered by RR Lyr becomes one of the
dominant contributors to the uncertainty in its absolute magnitude. Adopting
the average field absorption, =0.07 \pm 0.03, we obtain M_V^{RR} = 0.61
^{-0.11}_{+0.10}. This provides a distance modulus for the LMC, m-M = 18.38 -
18.53^{-0.11}_{+0.10} with the average extinction-corrected magnitude of RR Lyr
variables in the LMC, , remaining a significant uncertainty. We compare
this result to more than 80 other determinations of the distance modulus of the
LMC.Comment: Several typos corrected. To appear in The Astronomical Journal,
January 200
High angular resolution N-band observation of the silicate carbon star IRAS08002-3803 with the VLTI/MIDI instrument
We present the results of N-band spectro-interferometric observations of the
silicate carbon star IRAS08002-3803 with the MID-infrared Interferometric
instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) of the
European Southern Observatory (ESO). The observations were carried out using
two unit telescopes (UT2 and UT3) with projected baseline lengths ranging from
39 to 47 m. Our observations of IRAS08002-3803 have spatially resolved the
dusty environment of a silicate carbon star for the first time and revealed an
unexpected wavelength dependence of the angular size in the N band: the
uniform-disk diameter is found to be constant and ~36 mas (72 Rstar) between 8
and 10 micron, while it steeply increases longward of 10 micron to reach ~53
mas (106 Rstar) at 13 micron. Model calculations with our Monte Carlo radiative
transfer code show that neither spherical shell models nor axisymmetric disk
models consisting of silicate grains alone can simultaneously explain the
observed wavelength dependence of the visibility and the spectral energy
distribution (SED). We propose that the circumstellar environment of
IRAS08002-3803 may consist of two grain species coexisting in the disk:
silicate and a second grain species, for which we consider amorphous carbon,
large silicate grains, and metallic iron grains. Comparison of the observed
visibilities and SED with our models shows that such disk models can fairly --
though not entirely satisfactorily -- reproduce the observed SED and N-band
visibilities. Our MIDI observations and the radiative transfer calculations
lend support to the picture where oxygen-rich material around IRAS08002-3803 is
stored in a circumbinary disk surrounding the carbon-rich primary star and its
putative low-luminosity companion.Comment: 15 pages, 8 figures, accepted for publication in A&
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